放射线测定仪.doc

2.1 EXTERNAL CONTROLSThere are two external controls on the survey meter: an ON/OFF button and a MODE button.2.1.1 ON/OFF BUTTONPress the ON/OFF button to turn the unit on. All the elements in the display turn on and themicroprocessor runs through an initialization procedure. Part of the procedure includes reading thecalibration coefficients stored in EEPROM. If an EEPROM read error occurs, an error code (E1) will bedisplayed at power-on and unity calibration coefficients will be used.The bargraph and digital display will show a reading that decreases as the instrument stabilizes. Theinitial reading usually starts in the 50 R/h (50 mSv/h) range and decreases through the lower ranges to areading of less than 0.5 mR/h (5 Sv/h) within 40 seconds. When the elements in the display turn off,with the exception of those necessary for normal operation, the user can begin the measurementprocess.2.1.2 MODE BUTTONTo configure the MODE button to the opposite function without the RS-232 connection, use the followingprocedure:1. Turn the unit off.2. Press the MODE button.3. Turn the unit on while continuing to press the MODE button.4. Release the MODE button when the display is in the “all elements on” condition.5. Use the MODE button to toggle the instrument between the rate mode and the newly selectedfreeze or integrate mode.2.1.3 INTEGRATE MODEThe Integrate mode is operational 30 seconds after the instrument is turned on and all of the time afterthat. However the integrated exposure can only be displayed when the MODE button has beenconfigured as a toggle to display exposure rate/integrated exposure. If the MODE button is pressedwithin the 30-second initialization period of the unit, the display will read “0.”When integration starts, “0.0 R (0.00 Sv)” will be displayed. Toggle MODE to read exposure rate asrequired.To reset the integration display, toggle the display from the Rate mode to the Integrate mode. Keep theMODE button pressed for about 5 seconds. The display will clear, and then read “0.0 R (0.00 Sv).”Exposures are accumulated up to 999 R (9.99 Sv).2.1.4 FREEZE MODEWhen configured to select the Freeze mode, the MODE button acts as a toggle switch. Press the buttonuntil FREEZE appears on the display. Operation in the Freeze mode gives the user a constant referenceof the highest exposure rate obtained from the time the freeze function is initialized. The highest readingwill appear as a single bar on the bargraph. The current reading will continue to be displayed on thedigital display and the bargraph. If a measurement is obtained which exceeds the freeze bar reading, thefreeze bar will move to the higher measurement point. The operating range of the 451B remains lockedon the highest range attained during the Freeze mode so that the scale and the multiplier remain thesame.For example, assume the scale units appear as 10, 20, 30, 40, 50, and the freeze bar is at 47 mR/h onthe bargraph. If the 451B then measures a radiation field of 120 mR/h, the scale units will change to 100,200, 300, 400, 500, and the freeze bar will appear on the graph at 120 mR/h. If the survey metermeasurement goes below 100 mR/h, the units on the scale will not change until the survey meter is takenout of the freeze mode. However, the digital display will continue to show the current reading. Thesurvey meter will operate in the Freeze mode until the user toggles the MODE button to return to thenormal operating mode.If a measurement is obtained which causes a rangechange higher than the alarm set point range, the alarmwill sound and continue to sound until the freeze mode is2.2 SELF-TESTWhen the 451B survey meter is first turned on, it runs through a functional self test procedure. Duringthis self-test, the firmware version of the unit is displayed, along with the HI or LO chamber bias.If chamber bias is LO, the instrument cant measurehigh radiation rates accurately. Service to theinstrument is required.If the unit passes the self-test, it will go into the normal operating mode. If the unit fails the self-test it willremain locked with the firmware revision displayed. Consult a Cardinal Health, Radiation ManagementRepair Coordinator for corrective action.2.3 INSTALLATIONThe Model 451B periodically tests for mark signal at its RS-232 port. When an RS-232 signal is detected,the Model 451B sends test signals using its sending device and, if RS-232 is present and operational, thesurvey meter will enter into the communications mode. The 451B display will clear and the letters “CO”will be displayed as an indication that the unit is in the communications mode.The 451B is compatible with a CRT or printing terminal having a standard RS-232 connector and a 1200-baud rate. A computer with a modem or terminal emulator may be used in place of the terminal. Theterminal or computer should be set up for 1200 baud, 7 data bits, no parity bit, and 1 stop bit.1. Connect the 9 pin female connector to an active communication serial port.2. When “CO” appears on the 451B display, press the computer or terminal space bar twice.If the Model 451 displays the “CO” message but there is no response at the terminal after pressing thespace bar twice, perform the following tests:1. Be sure that the 451B and terminal are properly connected via the RS-232 cable.If there is still a problem, consult the Cardinal Health, Radiation Management Services RepairCoordinator for further instructions.2.4 OPTIONAL CHECK SOURCE USE (450 UCS)The check source contains radioactive materials,used for the verification of survey meter operation.Use care in the handling and storage.To check the Model 451B, perform the following:1. Turn instrument on and allow one minute to warm up.2. Open the beta shield.3. Place the check source flat against the end window.4. Check for a nominal reading of 1.0 mR/h (100 uSv/h).3.1 INTRODUCTIONThis unit contains lithium cells with a potential voltageof 63 V on the battery assembly. Use care inhandling this assembly during removal andinstallation. Damage to the instrument or bodily harmmay result.The user is cautioned about indiscriminately openingand disassembling the instrument. The highimpedance circuits of the ion chamber are easilycontaminated with grease and dirt that produceelectrical leakage. Complete Assembly/Disassemblyinstructions are available in the Maintenance Sectionof this manual.The Model 451B is an air ionization chamber instrument calibrated in exposure rate units ofroentgens/hour (or Sieverts/hour) for gamma and x-ray in the energy range of 20 keV to 2 MeV throughthe sliding Phenolic shield. It can also be used for x-ray in the energy range of 7 keV to 30 keV with thesliding shield open. The Model 451B responds to, but is not calibrated for, beta radiation, with the slideopen. Beta energies that can be measured are above 100 keV. The two thin mylar windows have acombined density thickness of 1.7 mg/cm2. The 451B also responds to alpha radiation above 4 MeV.The liquid crystal display shows the radiation rate in digital and analog form with the range multipliervalues also showing on the scale. It is a lightweight electronic device that requires the computationalcapabilities of a microprocessor to make it operate. It functions in a multiplex mode called quadruplex.This mode uses four backplanes to accommodate the 128 elements of the display.The microprocessor performs data collection, averaging, multiplication by stored calibration factors, rangechanging, and battery check functions, in addition to driving the LCD. Between computational periods, it“sleeps” in a low power mode to conserve battery power. The microprocessor reads stored informationfrom an electrically erasable memory, EEPROM, which is used by the program for calibration and displayunits. The EEPROM will retain stored data when the instrument is OFF or when the batteries areremoved. Data can be entered into the EEPROM using the RS-232 port.63 V collection voltage for the ion chamber is obtained from lithium cells that have a 10-year life. Allinternal power for the instrument is supplied by the 9 V batteries, accessible from the rear of theinstrument.The digital and bargraph displays read directly. The bargraph display update periods are listed in Table3-1. The digital display updates at one second intervals nearest the current bar display update. Thebargraph and digits display do not always show the same reading because the bargraph is faster than thedigital update. It is more convenient to watch the bargraph when the reading is changing quickly and toread the value of a slowly changing or static reading by looking at the digital display.Values can be displayed in SI units (Sv/h). This can be requested when the survey meter is ordered fromthe factory or it can be changed by the operator using the RS-232 port.The bargraph display is a digital presentation, programmed to appear as a linear analog meter display. Itis also referred to as the analog display throughout this manual.Bargraph display update periodsRange Update Period50 R/h (500 mSv/h) 0.05 second5 R/h (50 mSv/h) 0.1 second500 mR/h (5 mSv/h) 0.1 second50 mR/h (500 Sv/h) 0.15 second5 mR/h (50 Sv/h) 0.25 secondThere are 20 bars between each major division. The numerical values of the five major divisions changeappropriately for the range in which the instrument is operating. For instance, the first major divisionwould have the numeric value of 1, 10, or 100. The minor divisions are worth 0.05, 0.5 or 5. Theincremental nature of both the digital reading and the analog bargraph provide greater accuracy forreading in different portions of the scale. For example, on the 0-5 mR/h (0-50 Sv/h) range, with a digitalreading of 2.0 and above, the analog bargraph can be read more accurately than the digital display.Below a digital reading of 2.0, the digital display is more accurate because it consists of three significantdigits. The stated precision of the digital display is accurate only above a reading of 0.20 on the 0-5 mR(0-50 Sv) scale.The same analysis applies to all the other rangesbecause the number of significant digits or activebar elements are independent of the position of thedecimal point or the units multiplier.There is a small hysteresis built into the range changing circuit so that the instrument does not keepchanging scales if the reading is at the threshold of range change. It is important in calibration of theinstrument that the calibration coefficients track from range to range because an oscillatory condition canoccur if the calibration on a given range is low and the coefficient for the next more sensitive range ishigh.In addition to FREEZE, the other message that can appear on the display is LOW BAT. When the LOWBAT message appears, the batteries should be changed within one